October. 1925
IniD USTRIAL AND E-VGINEERING CHEMISTRY
1037
Artificial Silk’ With Special Reference to the Viscose Process By M. G. Luft TiiE
IXDUSl’RI.4L
FIBRECO INC.,CLEVELAND, OHIO
OLCTIOKS of baffling problems seldom result in radical changes in the methods of manufacture; nevertheless the chemist is seeking exact knowledge of what occurs in each step of a process. Such is the desire of the manufacturer of artificial silk, one of the youngest branches of the chemical industry. It is approaching its fulfilment but is still in need of assistance from chemical and mechanical research. Research work results in a better product obtained a t a lower cost, an achievement always desirable to a chemist. The manufacture of artificial silk is a good example of an industry in which the technic is considerably in advance of the theory of the process, which a t the present time is based largely on empirical research. The literature on the subject is very fragmentary, as most of the research so far has been carried out by the manufacturers and remains unpublished ; similarly, the patent literature contains little, if any, information of scientific value. Artificial silk represents a r t and refinement in textiles. The advance in a r t education and artistic taste called for radical changes in development of textiles. By the study of art in schools and encouraged by many publications, art museums, and organizations such as the Art and Industries Foundation, the educated portion of the population created a demand in recent years for more lively and artistic combinations of color, luster, and design of texture. The post-war period stimulated the desire for light and color, resulting in a renaissance of the textile industry. This, combined with the higher buying pon-er of the public and the increased knowledge of manufacture, is the source of the tremendous growth of the artificial silk industry. This new fiber is not a substitute for natural silk, but is R distinct textile product, as is wool, silk, cotton, or flax. We see how the silkworm produces the silk fiber, feeding on mulberry leaves, and this natural process we are reproducing in a chemical and mechanical way, using also cellulose as the raw material and spinning the solutions of the cellulose compounds. The worm changing from the caterpillar to the chrysalis stage in spinning its cocoon ejects through two extremely fine spinnerets in its mouth a semiliquid viscous substance which on leaving the glands hardens immediately upon contact with the air. It is composed of two albuminous substances containing about 80 per cent fibroin and 20 per cent sericin, the latter being a soluble gum.
S
cellulose solution was forced through minute holes into a precipitating bath and the fine filaments were wound in groups. This product was not yet denitrated and therefore very inflammable. The year 1891 witnessed the beginning of the manufacture of artificial silk on a commercial scale in BesanCon, France, by the Societ6 Anonyme pour la E’abrication de la Soie de Chardonnet. I n 1890 Despaissis dissolved cellulose in an ammoniacal solution of copper hydroxide, but it was not until 1897 that Pauly was able to start the production of cuprammonium silk. I n 1892 Cross, Bevan, and Beadle3 discovered sodium cellulose xanthate, the compound formed by the action of carbon bisulfide on mercerized cellulose, which they named “viscose” on account of its high viscosity. This discovery mas the start of the viscose process of artificial silk manufacture, which today is used more widely than any other. About 80 per cent of the total world production of artificial silk a t present is made from viscose and this growing industry is well established in the United States and Europe. About the same time Cross and Bevan produced the acetate esters of cellulose which became the basis of another artificial fiber known as acetate silk. Kinds of Artificial Silk These four processes are still used today, having in common cellulose derived from wood or cotton as a raw material. They also have in common similar manufacturing procedures -namely, chemical and textile operations. Tables I and I1 show the preparation of raw material and the four different kinds of artificial silk manufacture. Table I-Preparation of R a w Material COTTON S E E D SPRUW W O O D CI e a n 1ng Barking, cleaning, and Crushing Lnd pressing
I
Raw linters Digesting wiih caustic soda
Ri.aumur2 in 1734 suggested the imitation of natural silk by human artifice. This idea was forgotten until 1855, when the Swiss,Andemars, patented a process of spinning etheralcohol solution of nitrocellulose by dipping a pointed steel rod into the solution and pulling a thread. I n 1882 Swan, Wyne, Swinburne, and Powell produced nitrocellulose threads for carbon filaments for use in incandescent lamps, but their process was not commercially successful. I n 1884 Count Hilaire de Chardonnet conceived the idea of forcing a collodion solution through fine capillary openings and he patented a manufacturing process in the same year. The nitro1 Received March 6, 1925. P a r t of address delivered before the Chemical Club of Princeton University, January 29, 1925. * “Histoire des Insectes (1734-1742).”
I
Cooking in digrstors
I I
I
Washing and screening
I
Bleaching (Lime-chlorine)
I
Washing
I
Paper machines
Washing Bleaching (Lime-chlorine) Washing Drying BleachAd linters
History
chipping
Suifite (Sulfur-lime, calcium bisulfite)
Table 11-Kinds NITROCELLULOSE (Chardonnet silk) Purified cotton
I 1
Sulfuric and nitric acids Nitrocellulose Dissolved in alcohol and ether Filaments‘coagulated Nitrocellulose filaments ,
I 1 Removal of nitro groups I Recovery of solvents
Unbleached thread Bleaching
I
Commercial thread a British Patent 8700 (1892).
I 1
I I
Drying Cellulose (Cut in sheets)
of Artificial Silk CUPRAMMONIUM SILK (Also called Glanzstoff or Pauly) Purified cotton or wood pulp dissolved in ammoniacal copper solution Spinning ’solution
I
Coagulated by ,caustic soda or sulfuric acid
I I Bleaching I
Filaments of cellulose
Commercial thread
ZIEellllvC CII&.CIISTIZ Y
Vol. li, KO. 10
Acetate Silk
Coinmercial thread
I
vi$ccose sollltin" Spionin Ioi"ti0o
'i
Crude viscose Glameotr
Acetate silk is the acetic ester of celluluse. Ccllulosc is t,reated with acetic anhydride and glacial acetic acid in tlie presence of concentrated sulfuric acid and a catalyst. It is difficultto produce a homogeneous acetyl cellulose as the acetylation goes too far, resultiiig in a mixture of di- and triacetates. The acetate cellulose is then precipitated by water, washed until neutral, dried, and dissolved in acetone. After the filtrat,ion the solution is spun either in warm air or in a setting bath of water or a salt solution, the fornier process
ncmova1Iof ru1:ui nleachiiig I
Comrnrrcial thread
Nitrocellulose Silk
Nit.rocelluloso silk is made by t.reating cellulose in the form of cotton linters xvith a mixture of nitric and concentrated sulfuric acids. The resulting product, consisting largely of trinitrocellulose, is washed until neutral, hydro-extracted, dried slightly, and dissolved in a mixt.iire of alcohol and ether. This collodion solution after filtrstion is breed by pressure tlirough very fiiie glass nozzles. In tho dry system the oollodion on emerging from the spinneret loses the ether and alcohol by evaporation, leaving the formed filament of nitrocellulose. In the wet system the collodion is passed through a water solution known as a "setting bath." Several filaments are spun together and they form, after the twisting process, the thread consisting of nitrocellulose. The alcohol aiid ether eliminated from the solution by evaporation in the dry system, or by dilution iii the wet system, are partly recovered and used again as solvents for nitrocellulose. The resulting thread is inflaminable aiid is denitrated by a treatment with sodium hydrosulfide, leaving pure cellulose fibers, whicli are then washed, bleached, and reeled into skeins. The nitrocellulose process is quite expensive on account of the high cost of the cliemicals used. If the denitration is not perfect the silk beeorncs yellow when stored and does not take the dye evenly. Cuprammonium Silk
In the cuprammonium process the copper liydroxide unites with the eellulose to form an adsorption compound which is soluble in ammonia. The resulting product has the following composition: 2.5 to 3 per ceiit copper, 7 to 8 per cent ammonia, and 7 to 8 per cent ecllulose. This process was developed on a commercial scale about 1'300 arid was used largely in Germany for I'auly or Glunestoff silk. The solution of cellulose in ammoniacal copper oxide is unstable and must be kept at low tcniperatures approaching tlie freezing point of water. It is spun in a manner similar t.o that of nitrocellulosc by tlic wet system, the solution bcing forced through fine orifices or spinnerets into a setting bath of dilute suliuric acid. I t caii also be spun in a caustic soda soliition and tlie copper rernoved afteiwards from the fiber by treatment with dilute sulfuric acid. The copper silk is then bleached and purified until a product is obtaiiied which is very soft to the toitch and possesses high luster. Most of tlie copper silk plants have gradually changed over to the viscose process. I n the United States several attempts have been made and are still being made to manufacture artificial silk by the copper process, hut as yet they have not been successful. The last company in England to use this process was the Kent Silk Mills, which has abandoned it recently aiid has clanged to the viscose process, claiming that the cost of manufacture is about 60 per cent less.
F i s u r e I --Hydraulic I'rremes for the MerCerizetlctn of Cellubee
allowing the recovery of the expensive so1vent.s. The resulting fiber is not, a regenerated cellulose, but an acetate of cellulose which does not adsorb moisture so readily as the other kinds of artificial silk. Therefore, it has less affinity for dyes, this property prohibiting, so far, its use on E large scale in the textile industry. Acet,ate silk is sold under the trade names Celmese and Lustron. Viscose Silk
The raw niaterial used in the viscose process is bleached spivce pulp, bleached cotton linters, or a mixture of the two. The silk made from pulp has a mild, not opalescent luster, and dyes more uniformly than thresd made from cotton, because the latter is a more highly polymerized body and therefore more resistant and less active than bleached sulfite pulp. The pulp, cut in rectangular sheets, is soaked in an 18 per cent caustic soda solution. This treatment causes tlie fibers to swell and an unstable coinpound of soda and cellulose is formed wbioh is known as soda cellulose. The excess of caustic soda solution is removed by means of hydraulic prcssure (Figtire 1). It is very important to maintain tlie concentration and temperature of the solution and the time of soaking the same from batch to batch t o obtain a uniiorm product. Also, the ainount of caustic soda solution remaining in the cellulose should be constaiit.-viz., about two parts of solution to one of pulp. Thereafter tho sheets of soda cellulose are placed io shredders, the rotating blades of which operating against a grating grind t,he soda cellulose into a fine crumbly state. (Figure 2 ) The heat developed in the shredding of soda cellulose is removed hy circulating brine through the jacket of tlie shredder, thus maintaining the soda cellulose at a constant temperature. The shredded material is conveyed to small containers and undergoes a so-called aging process at a constant temperature for a period of time. The soda cellulo.;e has an increased affinity for dyes and greater reactivity than cellulose, but it is an unstablo compound,
the charactcr and the velocity of the decomposition being a function of time and temperature. Table 111 illustrates the change that soda cellulose has undergone during a period 01 observation of over one year. The two samples were kept at a constant temperature in air-tight containers and a t various intervals analyses were made.
tic soda present in the viscose forins a t tire same time byproducts, as shown in Reactions 3, 4, and 5. 3CB
+ +6NaOH = 2NarCSs+ NazCOi + 3H20 3NaOH = 3NaHS + NazCOa
NasCS
Cs1
+ 2NaHS = Na,CSa + H a
(3) (4)
(5)
The reactions are continuous. The by-products-sulfides, polysulfides, and thiocarbonates-are further decomposed into hydrogen sulfide and free sulfur by the acid solution in ___-~ ~ - - the precipitating bath in the next step of rnaiiufacture where e. Hemim. €lemi. Time of Cellu- ce11uCellu. rcllrrviscose is transformed to cellulose filaments. The viscose aging NaOH NaCQ lose lose NaOH Nnd201 lose lose solution is a gel which can be controlled by dialysis, one of the iday 14.3 0.72s.co.4 14.1 0 . s z s . c 0.8 2weeks 13.4 1 . 2 2 8 . 1 2 . 3 13.4 1.328.0 1.9 many interesting problems of colloid chemistry connected with 12weeks 9.3 4.0 23.9 4.2 84 weeks 1.2 1 1 . 4 33.5 14.3 the ripening process of viscose solution. The transition of 72aeeks 1.3 13.0 15.7 12.5 the cellulose xanthate from the relatively small molecular to the semiliquid state whereby the small molecondition In the aging process the uniformity of temperature, time of aging, and size of the containers are important factors, as they culcs are polymerized is a change that is visible under the determine certain properties of the resulting viscose and the ultramicroscope. During the ripening the viscose is filtered subsequent steps of the manufacture. The aging of soda several times to obtain a nniform solution and to remove cellulose can be accelerated by oxidation and there are several impurities such as iron, undissolved fibers, etc., which would methods known which make use of catalysts for this purpose.4 block the fine holes in the spinnerets. The point where the XAm€iAiPoN--After the soda cellulose has been properly viscose is sufficiently ripened and ready to be spun is deaged it is transformed into xanthate of cellulose, a reaction termined for each batch by a simple laboratory method of similar to ester formation of cellulose. This operation takes precipitation. At this stage the cellulose xanthak is polymplace in double-jacketed, air-tight containers (Figure 3), erized and, being less soluble than when freshly prepared, where the soda cellulose is acted upon by carbon bisulfide as it can be decomposed and solidified in the spinning process, the barrels rotate slowly, and the excesa heat of this exother- delivering a uniform product. Finally, the air bubbles a+ mic reaction is removed by cooling brine circulating through cumulated in the viscose are removed hy vacuum and the the double jacket. Length of time and uniform tempera- purified solution is transferred as a continuous Bow to the ture are also importaut factors of the xanthate reaction. spinning machines to be converted into filaments. During this process the white soda cellulose is changing color, which indicates the progress of xanthation. The final product of the reactiou is a sticky, amorphous mass of orange color soluble in water. However, the pure xanthate of cellulose is an amorphous, colorless compound, the by-products containing sulfur imparting the yellow-orange color. The reaction of xanthation is shown by Equation 1. Table III---Effect of&tins2 .. . (aia"res in per ce"t) sampia I--Sample 2-
--
CeHIO,.ONa
+ C S = NaS.CS.OCeMeOO.
~
(1)
The resulting cellulose xanthate is readily soluble in caustic soda iving a viscous orangecolored solution called in the industri 'viscose." The product is labile and is decomposed by acids, acid salts, and ammouium salts, nr if left by itself for a long time i t is regenerated to cellulose hydrate. This reaction continues until equilibrium is reached, which takes place quantitatively under special working conditions. The decomposition is spontaneous, the by-products after the formation of cellulose hydrate being caustic soda, carbon bisulfide, and polysutfides. The freshly prepared cellulose xanthate is transferred frnm the tumbling barrel to a mixer containing dilute caustic soda. The mixers are iron tanks equipped with revolving arms, and by means of a cooling device the solution is kept at a low, uniform temperature. After the solution of the xanthate is completed, the liquid is pumped to the ripening department, where it remains a t a low, uniform temperature lor a period of'time until the ripening of the viscose is effected, a process similar to the aging of soda cellulose described before. It undergoes polymerization, during which the xanthate forms larger complexes by splitting off the sodium snd sulfur compounds (Reaction 2).
During the aging process the solution of viscose shows a progressive reduction nf polarization in presence of increasing alkali concentration. The excess of carbon bisulfide and caus1 British
P.tentsi8,W3 (1914); 14,875 (1914).
Figure 2-Shredders
S l ~ I N N l N ~ u n d e uniform r pressure, undergoing final filtration, the viscose is delivered through mechanical regulators or pumps placed on the spinning machines, each regulator pumping a constant amount of viscose to the outlet fitted with a platinum spinneret. The solution forced t.lirough the minute lioles of t,he spinneret is divided into very fine streams which, passing through the precipitating bath, arc immediately coagnlated and the streams of viscose solution are converted into fine filaments of regenerated cellulose. The reaction that takes place when the alkaline viscose solution is spun or precipitated occurs in two stages: first, the prccipitation rrf tho cellulose xanthate caused simultaneously by the salts present in the setting bath and the neutralization of the excess of alkali by the acid also present in the bath; and second, the decomposition of the cellulose xanthate to cellulose hydrate by the prolonged action of acid. The semiliquid state of the coagnlated product allows the for-
I N D U S T R I A L A N D ENNGINEERING CUE.IfIsTRY
1040
mation of filaments which in diameter correspond wit,Ii the minute holes of the jet through which the solution of viscose has been ejected under pressure. During the precipitation an elevated temperature of the spin bath is desirable in order to accelerate the coagulation. There is also a relation lietween the surface tension of the thread aiid the conrmtration of the coagulating medium. The size of the spun thread is determined by the amount of viscose solution of known cellulose content projected through the jet in unit time, and t,he rate a t wliich t.he thread is wound on the spool or collected in the centrifuge box, which usually is about 50 meters per minute.
Figure 3-Tumbling Barrels
The result of the spinning process is the formation of insoluble cellulose hydrate, this formation being directly proportional to the strength and temperature of the acid in the precipitating bath and to the time of immersion. During the spinning the hydrated cellulose complex while under tension changes the aggregate of the molecules in p acids, resulting finally in a n amorphous structure the hydration the cellulose filaments are swellin nomenon similar to the previously described procem of mercerization. The spinning reactions of viscose can be outlined in the final stage in the following way: 2s
= i
C
